This invention relates to a retaining arrangement for rotor blades of axial-flow turbomachinery according to the generic part of claim 1.
Various retaining mechanisms for rotor blades of axial-flow turbomachinery are known in the prior art. Generally, the rotor blades of a turbine stage are fixed in slots provided for this purpose on the circumference of a rotor disk. In these designs, the blade roots are drop-shaped or of the dovetail-type to enable the individual blades to be easily fitted axially into corresponding slots on the circumference of the rotor disk. The blade roots are tapered to retain the individual blades in the rotor disk in the radial direction. To prevent the rotor blades from being displaced in the axial direction, set screws are used, for example, to secure them in their location. In other retaining mechanisms, clamping pins are provided to produce a corresponding fixation in the axial direction.
A further retaining arrangement for rotor blades known in the prior art features slots with firtree-shaped serrations on the circumference of a rotor disk. Into these slots, the corresponding blade roots, which also feature a firtree-shaped serration, are fitted in the axial direction. This fixation arrangement is slightly conical in the axial direction so that the blade can fitted or removed in one direction only. To prevent the blade from unintentionally getting loose and detached from the blade root location, the blade root according to this state of the art is secured in its position by locating plates.
Such locating plates are taught in Specification EP 0761930 A1, for example. These locating plates are costly in terms of manufacture and require special tooling for assembly. The high mass of the locating plates entails high centrifugal forces.
In order to improve the flow conditions between the individual blade stages, Specification U.S. Pat. No. 4,846,628 proposes a continuous, circumferential locating and sealing ring which features projections on its inner circumference, these projections engaging corresponding jaws on the rotor disk. This locating and sealing ring covers the entire blade root height and has a labyrinth-type seal on its outer circumference. Furthermore, the locating and sealing ring described in said Specification and the blade roots must be positioned precisely relative to each other in the axial direction to enable the circumferential contact seal provided on the locating and sealing to become effective.
A similar locating and sealing ring 90 according to the state of the art is shown in FIG. 9. In the design according to this state of the art, assembly and fixation is accomplished similar to a bayonet lock. Since the locating and sealing ring used in this arrangement also serves as a seal against leakage flow between the rotor 93 and the blades 94, it virtually covers the entire blade root 91 and, due to the circumferential contact seal 92, must be manufactured and assembled with high accuracy. Furthermore, this locating and sealing ring 90 features a labyrinth-type seal 95 on its outer circumference.
The two latter locating and sealing arrangements entail the disadvantage that they are very expensive in terms of the exacting manufacturing and assembly tolerances. The high mass entailed by these arrangements compromises the achievable rotational speed and life, respectively.
Specification EP 0463955 B1 teaches an embodiment providing a circumferential retaining ring for rotor blading, this circumferential retaining ring covering the major part of the blade root. However, neither the design nor the method of functioning is further described for this state of the art.
In a broad aspect, the present invention provides a retaining arrangement for rotor blades of turbomachinery which avoids the disadvantages of the state of the art. More particularly, the present invention relates to a cost-effective retaining arrangement which provides for better control of the leakage flow in the blade root area, generally improved flow conditions in the inter-stage area, smaller centrifugal forces and ease of assembly.
It is a particular object of the present invention to provide a retaining arrangement according to the features expressed in claim 1. Further advantageous aspects of the present invention are cited in the subclaims.
The retaining arrangement for rotor blades of axial-flow turbomachinery with the features of claim 1 according the present invention provides for cost effectiveness, improved control of leakage flow in the blade root area, generally improved flow conditions in the inter-stage area, lower centrifugal forces and ease of assembly. Since the locating function which is provided by the retaining ring according to the present invention is separated from the sealing function which, by way of the retaining ring according to the present invention, is provided via corresponding sealing faces between blade and rotor disk, a substantial saving in mass is achieved. In addition, the omission of contact or labyrinth-type seals permits larger manufacturing and assembly tolerances. In this design, the retaing function is provided by projecting sections on the outer circumference of the retaining ring, these projecting sections acting together with corresponding blade root sections. The following direction indications assume the rotor disk to be the reference system.
In alternative developments of the present invention, the projecting sections provided on the outer circumference of the retaining ring may be designed either according to claim 2 or according to claim 4. The design of the projecting sections governs the design of the corresponding blade root sections according to claim 3 or according to claim 5. Other than in the embodiments of claim 2 and 3, the projecting section provided on the outer circumference of the retaining ring according to the embodiment of claim 4 and 5 is fitted sidewards into a corresponding location on the blade root. Since the retaining ring can easily be fitted with conventional assembly tooling, the present invention dispenses with the expenditure for special tooling.
In a beneficial embodiment of the present invention with the features of claim 6, a locking arragnement which functions similar to a bayonet-type lock is provided for the retaining ring.
The embodiment of the present invention according to claim 7 is particularly advantageous. This embodiment provides for a considerable saving in mass which reduces both the manufacturing costs and the centrifugal forces and enables higher rotational speeds to be achieved. In addition, the reduced coverage of the blade root enables flow ducts to be provided in the blade root area which, by passing through the blade root area, improve the flow and cooling conditions between the rotor stages.
In an advantageous embodiment of the present invention with the features of claim 8, a locking block may be provided, for example, which is fitted between two hooks of the locking arrangement. The locking block may be secured against unintentional detachment by way of lockwires, for example. For redundancy reasons, several locks of this type are normally provided on a rotor stage.
The design of the blade/disk sealing surface according to claim 9 dispenses with additional seals on the retaining ring according to the present invention. The manufacture of the sealing faces at the bottom of the blade platform and the rotor lobes does not require additional operations since it can be accomplished in the course of re-machining of the respective parts. Inaccuracies in the axial positioning of the blades do not adversely affect the sealing performance in this embodiment.
The design of the blade root slot in accordance with the features of claim 10 is particularly advantageous. In particular, a serration of the firtree type which tapers in the axial direction is favorable. Moreover, the retaining arrangement here described is suitable for both compressor and turbine stages. Its application in a turbine stage is, however, particularly advantageous.